This invention relates to a glow discharge lamp for qualitative and quantitative spectrum analysis in general and more particularly to an improved glow discharge lamp of the type having a discharge chamber closable on the cathode side by an electrically conductive disk at cathode potential.
In analyses used, for example, to monitor production or for routine material control, there is a requirement to determine the components of a substance or the concentration thereof quickly and at little expense, with a narrow error range. Often only small amounts of the substance are avaiable.
For such analyses, optical emission spectrum analyses making use of the cathode glow of the substances to be examined has proven especially favorable. Generally a rare gas, preferably argon, is used as the working gas for producing the glow discharge.
A known glow discharge lamp for producing the cathode glow and acting as a light source has an anode body which possesses, on the cathode side, an electrically conductive tubular projection connected with the anode body, and has a disk of electrically conductive material pressed against the side of the cathode body surrounding the tubular projection, away from the anode, the distance between the tubular projection and the practically plane surface of the disk being 0.05 to 0.5 mm. In this glow discharge lamp, the working gas is pumped off through the gap between the tubular projection and the surface of the disk. The disk may itself consist wholly or partly of the substance to be analyzed or, if the latter is liquid, be impregnated with it. Alternatively the substance to be analyzed may be placed on the disk or, if it is gaseous, be admixed to the working gas (U.S. Pat. No. 3,543,077). Further the disk may be provided with one or more bores to receive wire type samples to be analyzed (U.S. Pat. No. 3,626,234).
In the operation of this known glow discharge lamp a cathode glow of high luminous intensity, forms over the disk at cathodic potential, which glow is limited by the bore of the anode tubular projection. The substance to be analyzed, continuously atomized by bombardment with working gas ions or already admixed to the working gas in gaseous form, is excited to emit light in the cathode glow which can, in this state, be detected by spectrum analysis and determined quantitatively.
With increasing luminous intensity, that is, increased excitation of the atoms of the substance to be analyzed, the accuracy of the analysis increases. To achieve such an increased excitation, a high current low voltage discharge has previously been provided in a glow discharge lamp of the known kind, in addition to the glow discharge. Through this additional discharge, the electron density in the discharge chamber and hence the number of collisions of the electrons with the atoms of the substance to be analyzed is increased. Due to this additional discharge, however, the construction of the glow discharge lamp becomes relatively complicated since two additional electrodes are necessary. These are sealed in glass tubes communicating directly at the discharge chamber. (Spectrochimica Acta 31 B (1976), pages 257 to 261). It is also known, in hollow cathode lamps, to divide the cathode into two half-shells which are electrically insulated and held together by a layer of adhesive. The inner cathode half-shell can be designed as a control electrode as seen in Offenlegungsschrift No. 1,905,318, Aug. 6, 1970. However, such a control electrode, because of the discharge conditions due to the hollow shape of the cathode, has little influence on the intensity of the spectral lines.
Furthermore, glow discharge lamps for optical spectrum analysis having a discharge chamber which can be closed off on the cathode side by an electrically conductive disc at cathode potential, have already been proposed in which means for producing a magnetic field extending in the discharge chamber subsantially in the axial direction are provided to increase the light intensity and the accuracy of the analysis. These means may consist, in particular, of a ring magnet of permanent magnet material magnetized in the axial direction as disclosed in my copending Application Ser. No. 032,009, now abandoned. A controllable electromagnet can also be disposed coaxially with this ring magnet as also disclosed in my copending Application Ser. No. 032,009. Through appropriate control of the electromagnet, for instance, to a constant discharge, analysis errors due to differences in the samples can be reduced.